1. Field of the Invention
The present invention relates to a tyre for motor vehicles, and more particularly to a tyre for heavy motor vehicles such as lorries and similar, used both on roads and on rough ground such as that generally present in quarries and/or on building sites.
More particularly, the present invention relates to tyres to be used on the driving axles, generally the rear axles, of the aforesaid vehicles.
2. Description of the Related Art
In the remainder of the present description and in the following claims, the expression “elastomeric material” is intended to denote a rubber mixture as a whole, in other words one containing at least one base polymer, reinforcing fillers, and further process additives such as accelerators, anti-ageing agents and crosslinking agents used for manufacturing and curing the tyre.
As is known, medium and heavy motor transport vehicles, such as lorries, are generally fitted with tyres whose types differ according to whether they are to be fitted on axles of the driving type or of the driven type, this differentiation being due to the difference in performance and behavioural characteristics required from tyres according to the axles on which they are fitted.
For example, the tyres fitted on the driven axles are primarily required to nave a high directional stability, low abradability, to provide good mileage, and high tear resistance, together with a good grip and lateral roadholding or rough ground, for example on muddy and/or sandy ground.
To achieve these objectives, use is made of tyres having a tread of the type called “grooved”, in other words a tread provided with grooves of the circumferential type, with a rectilinear or zigzag configuration, which form on the tread a plurality of circumferential ribs, in other words a plurality of continuous circumferential ribs which normally do not have transverse incisions extending over their whole width.
On the other hand, tyres fitted on the driving wheels have a tread of what is known as the “block” type, since a plurality of blocks is provided, formed by a combination of circumferential grooves intersected by transverse grooves, of considerable width; the dimensions and orientation of these transverse grooves have a considerable effect on the gripping action exerted by the tread on the ground.
Tyres fitted on the driving axles are generally required to have high traction and good roadholding, particularly on loose ground such as that normally present in quarries and/or on building sites.
A further requirement, increasingly manifested on the market and in the legislation of various countries, is for a significant reduction in the noise threshold of the tyre during its normal use on road routes. In this respect it is known that tyres of the “block” type generally have a higher noise level than tyres of the “grooved” type.
A tyre for medium and heavy motor transport vehicles is also generally required to have high regularity of wear, a good mileage, and high tear resistance, together with a tread design capable of effectively expelling stones and/or debris which may be trapped within the grooves formed in the tread.
This is because the continued presence of such stones and/or debris may cause perforations in the tread and, over a period of time, give rise to irreversible damage to the underlying belt layers.
An example of the documents known in the prior art and relating to tyres of the “block” type for medium and heavy motor transport vehicles is Italian patent No. 1,245,773 in the name of the present applicant.
This patent describes a tread comprising at least four rows of blocks delimited by at least three circumferential grooves of different widths, the narrowest two being located laterally, on opposite sides of the equatorial plane of the tyre, in an axially outward position, and by a plurality of oblique transverse grooves connecting adjacent pairs of circumferential grooves.
In this embodiment, the narrow groove separating the two lateral rows of blocks on each shoulder of the tyre is characterized in that it has a width of not more than 2.5 mm and a depth not exceeding that of the oblique transverse grooves, while the blocks of the two lateral rows are staggered circumferentially with respect to each other, so that each block of each row is next to two consecutive blocks of the adjacent row.
In a different embodiment described in this document, two additional, axially inner, circumferential grooves are also provided; these delimit a circumferential rib on whose sides are formed oblique transverse grooves extending to the equatorial plane, in such a way as to impart to this rib the appearance of two adjacent rows of blocks.
The blocks of all the rows have an ordered configuration, in that all the longitudinal sides of the blocks belonging to a given row are positioned in two planes parallel to the equatorial plane.
In a further embodiment, the two rows of blocks of the central rib are joined together to form a single row of blocks passing through the equatorial plane. In a further embodiment, the blocks of the central rib have an essentially hexagonal shape.
Another tyre for lorries, known from U.S. Pat. No. 4,412,574, has a tread which comprises two blocks adjacent in the transverse direction, one being longer than the other, and such that the shorter block follows the longer one, followed in the circumferential direction by another two adjacent blocks whose order is the reverse of the preceding ones: in other words, the longer block follows the shorter block. This configuration is repeated over the whole longitudinal extension of the tread. These pairs of blocks are delimited by transverse grooves which extend between the shoulders of the tread.